Reply To: Quantum causal models, faithfulness and retrocausality (onl. 7/16 @ 11pm UTC+10)

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Peter Evans

Hi Mark,

Let me start by saying that I don’t disagree with anything you’ve said there. But I also don’t think you’ve quite understood my point. I’m not trying to advocate a position wherein we need to add some dynamical element to our 4D description in order to satisfy our temporally biased perspective; in fact, quite the opposite. Let me try to explain…

Let us take it as given, for the sake of this discussion, that reality is best understood as a 4D block that obeys a global constraint of the sort you advocate (and a few others on this forum also advocate). If this is indeed the case, then since it is also the case that we occupy this reality and we have been able to provide rather successful causal and dynamical models representing the phenomena around us, then there must be some sort of story explaining how we can do this given that reality is actually a 4D block obeying said constraints.

But we don’t do this by giving some competing dynamical story in terms of time evolving laws. What we’re after is a dynamical story that arises as a result of a combination of the global constraints and our spatiotemporally embedded perspective.

If the global constraints were of a sort that gave us a kind of Newtonian reality, then the task of telling that dynamical story would be trivial: the 4D picture would be one that was consistent with the sort of time evolving laws that we usually take to describe/explain ordinary phenomena. But we know that there are significant problems created for this sort of dynamical view by the EPRB correlations; time evolving laws just won’t “save the appearances”.

When the global constraints contain final boundary conditions, the appropriate dynamical picture that we tell about our local experience of the phenomena will involve some sort of retrocausality. But we shouldn’t need to introduce any fancy new dynamical mechanism (like the pseudotemporal processes you mention), we just take note of the global constraints, and our particular perspective, and take these into account when enumerating our theory describing the phenomena. In particular, our spatiotemporally embedded perspective is going to mean that we have epistemic access only to initial boundary conditions and so we’re going to be providing some sort of probabilistic story based on these and in the absence of any final boundary conditions.

That this story is dynamical is a feature of how we tell it, not as a result of some objective “dynamicism” in reality. And I think this is the most fruitful way to understand retrocausality.

And just a last point on fine-tuning. Faithfulness is an assumption that statistical independences shouldn’t hide causal dependences—and if they did, then they would have to be fine-tuned to do so. Thus the fine-tuning under consideration is of the causal mechanisms relating variables such that they hide the statistical dependence that we would expect to see from such mechanisms. I think this might be slightly different from some claim that rules out ad hoc causal mechanisms.

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